Built-in type household appliance appliances in use today have a treatment chamber. In a household appliance in the form of a cooking appliance, the treatment chamber is also referred to as cooking chamber and serves to receive food to be cooked.
In various cultures, especially in the Asian culture, and very particularly in the Chinese culture, it is known to cook foods with hot steam. This way of preparing foods is referred to as steam cooking or steaming and is done using so-called steam cookers.
In a steam cooker, the foods to be cooked are exposed to steam of boiling water. When steam cooking at ambient pressure, the cooking temperature is 100° C., and when pressure steaming, the temperature is typically 120° C. For this purpose, water is evaporated inside the steam cooker by a heating device and supplied to the treatment chamber which holds the foods to be cooked, which are then cooked by the steam.
During the cooking process, the water partially condenses, for example, on the inner side of the treatment chamber, and collects as condensate at the lowest point of the treatment chamber. Thus, this condensate, or condensed water, is no longer available for the steam cooking process. Therefore, it is known to provide an additional heater in this region for re-evaporating the condensate collected there. The object of this is to reduce condensate, to increase the volume of stream, and to reduce the duration and cost of the cooking process.
A heating device for evaporating condensate from an air conditioner is disclosed in U.S. Pat. No. 6,965,732 B2. The heating device shown is disposed in a treatment chamber and composed of a plurality of resistance heating elements, which are covered on opposite sides by planar electrodes.
It is known from steam cookers of the Miele company to provide an opening in the bottom in the lower region of the treatment chamber, and to insert a heating element therethrough to evaporate the condensate.
The disadvantage here is that this opening for passage of the electrical leads of the heating element must be provided with a seal to prevent condensate from leaking downwardly out of the treatment chamber at this location. This increases the complexity of assembly and adds to the material costs. Moreover, the durability of this seal may constitute a weak point for the long-term durability of the steam cooker.
Another disadvantage is that this arrangement is not impervious to microwaves, and therefore cannot be used with combined appliances which include both a steam cooker and a microwave.
In order to make this additional heater suitable for use in microwave applications, an electrically conductive connection may be created between the treatment chamber and the heating element using a wire mesh, so that no radiation can leak out through this opening during microwave operation. However, this wire mesh makes manufacture more complex and correspondingly more expensive. Moreover, in the case of long-term use, for example over a 20-year lifetime of the appliance, there is a risk that the electrical connection between the treatment chamber and the heating element may deteriorate because of corrosion due to leakage or oxidation and that radiation may leak out.
In order to avoid these drawbacks, it is known from the combi-steam cookers with microwave of the Miele company to provide a foil heating element for evaporating condensate below the lower region of the treatment chamber, where the condensate can collect. More specifically, the foil heater is disposed under the bottom of the treatment chamber in this region and is typically composed of the following layers: self-adhesive film, silicone, glass silk, etched resistor layer of metal, and silicone. This makes it possible to heat the bottom of the treatment chamber across the surface thereof and to effectively evaporate the condensate.
A disadvantage here is that when the condensate has evaporated at any point of the foil-heating element, the temperature of the foil-heating element may increase at this point to such an extent that the foil-heating element destroys itself.
In order to avoid this, a PT-1000 temperature sensor (platinum measuring resistor) is mounted at the highest point of the three-dimensionally adhesively attached foil heating element, so that the temperature of the foil heating element can be monitored at this point. If an excessively high temperature of, for example, about 145° C. is sensed at this point, the electronics of the combi-steam cooker with microwave may cut off the electrical power to the foil heater. Since generally this highest point of the bottom of the treatment chamber is the first to be dried by the foil heater, it is thus possible to protect the foil heater from overheating. In addition, a thermal cutoff is mounted at another position of the bottom of the treatment chamber. The thermal cutoff is also capable of protecting the foil heating element from overheating. Such a foil heater is disclosed in German Patent Publication DE 10 2014 112 589 A1, this foil heater having a safety device which is attached to or associated with the foil heater and serves to prevent overheating of the heater and/or the bottom.
German Patent Publication DE 195 23 301 A1 describes a heater for a high-temperature metal oxide sensor having a conductive heating track and a conductive measuring track. The conductive heating track is U-shaped and encloses in its interior the conductive measuring track, which is also U-shaped.
The disadvantage here is that the heating element can be protected from overheating only at certain points. Therefore, if the appliance stands in an inclined position or if the treatment chamber was mounted non-parallel to the treatment chamber bottom, it may occur that a different point of the foil heater is dried first and that this cannot be detected, so that the foil heating element may destroy itself despite measuring the temperature.
Another disadvantage is that the maximum temperature of the foil heating element, which is about 140° C., is limited by the self-adhesive layer of the foil heating element. This low temperature resistance of the self-adhesive layer requires a low power density of the foil heating element in order to protect it from overheating. As a result, the heating element must have a relative large surface area to be able to provide the heating required for drying the condensate. This, in turn, may increase the manufacturing cost or result in slower evaporation of the condensate. Furthermore, this may increase the amount of residual condensed water.
From the combi-steam cookers with microwave of the V-Zug company, it is known to provide an assembly including a plurality of positive temperature coefficient (PTC) heating elements under the bottom of the treatment chamber. The PTC heating elements have the characteristic that their electrical resistance increases with increasing temperature. This enables the PTC heating elements to self-limit their power input and thus their further heating without having to take additional measures. Furthermore, the individual PTC heating elements can limit themselves independently of each other; i.e., it is insignificant which of the PTC heating elements is free of condensate first.
However, the disadvantage here is that the heat transfer from the PTC heating elements into the bottom of the treatment chamber is relatively low compared, for example, to a foil heating element that is adhesively bonded to the underside of the treatment chamber bottom over a large surface area thereof, so that only little heat output can be transferred to the bottom of the treatment chamber. This effect may be aggravated by the fact that the energized PTC heating elements are wrapped in Kapton film for electrical insulation, which may hinder the transfer of heat.
From the steam cookers of the Backer company, it is known to heat the region of the treatment chamber bottom where condensate can accumulate using a foil heating element having small, parallel-connected heating elements which are self-controlled by a PTC effect.
The disadvantage here is that only a temperature range of up to about 70° C. can be used in this manner.